Composite structures are used in a wide variety of applications due to their high strength-to-weight ratio, improved corrosion resistance, and other favorable properties. In aircraft construction, composites are used in increasing quantities to form the fuselage, wings, horizontal and vertical stabilizer, and other components. Composite articles may be formed by laying up composite plies comprised of reinforcing fibers impregnated with polymer matrix material. The composite plies may be laid up on a forming tool having a tool surface contour matching the desired contour of the cured composite article.
In some cases, a caul plate may be positioned on top of the composite layup such that the composite layup is sandwiched between the caul plate and the forming tool. The caul plate may impart a desired contour and surface finish into the surface of the composite layup. Heat and/or pressure may be applied to the caul plate and/or composite layup to promote resin cure and to reduce the resin viscosity to facilitate resin flow such that the resin in the composite plies may intermingle.
In some examples, a release coating such as Frekote™ may be applied to the tool surfaces prior to layup of the composite plies. A release coating may also be applied to the caul plate surfaces prior to positioning of the caul plate on top of the composite layup. The release coating may prevent bonding between the composite layup and the forming tool and/or caul plate surfaces to allow the composite article to be removed from the forming tool after cure. Unfortunately, the release coating may result in non-uniform wetting of the surfaces of the composite layup which may result in the occurrence of surface porosity in the cured composite article.
Attempts to reduce the occurrence of surface porosity in the composite article include the use of peel plies which may be applied to the tool surface of the forming tool prior to layup of the composite plies on the tool surface and/or prior to the application of the caul plate over the composite layup. In some examples, peel plies may be adhesively-bonded to the surfaces of the forming tool and/or the caul plate. The peel plies may promote the uniform wetting of the surfaces of the composite layup during cure and may thereby reduce the occurrence of surface porosity in the cured composite article.
Unfortunately, the application of the peel plies to the tool surface of the forming tool and/or caul surface of the caul plate is a time-consuming process. In addition, the peel plies must be replaced after each cure cycle which may present challenges due to difficulties in separating the adhesively-bonded peel plies from the forming tool or caul plate. Furthermore, the surfaces of the forming tool and caul plate must be cleaned and prepared for the re-application of peel plies prior to each new cure cycle. In some examples, the peel plies may cause the formation of wrinkles in the outer surfaces of the cured composite article and which may require time-consuming rework.
As can be seen, there exists a need in the art for a system and method for reducing or preventing surface porosity in composite articles that avoids the drawbacks associated with peel plies.